Bis-pentadienyl complexes

Bis(oxazolinyl)phenylrhodium complex, in styrene asymmetric hydrosilylation, 10, 821 Bis(pentadienyl) complexes preparation and properties, 5, 336 with titanium(II), 4, 265... 

In non-aqueous electrolytes, the different properties of the solvated metal ions lead to different equilibrium and standard potentials. For comparing standard potentials, electrode reactions should be defined as reference systems with similar values in different solvents. Koepp, Wendt, and Strehlow suggested ferrocene/ferrocinium and cobaltocene/ cobaltocenium redox systems. The redox systems are bis-pentadienyl complexes of Fe +/Fe + and Co /Co , respectively. Gritzner and Kuta recommended ferrocene/ferrocinium and bis(biphenyl)Cr(l)/bis(biphenyl)Cr(0). Salt bridges with conventional cells should be avoided. Similar to aqueous electrolytes a reference to the physical potential scale is possible. Similar considerations hold for ionic melts and molten and solid electrolytes. 

The preparation of pentadienylsodium was being studied at about the same time as ferrocene was discovered. It was not until 1968, however, that the first binary pentadienyl complex of a transition element, bis(pentadienyl)-chromium, was obtained from PINa and CrCl2 (121). This compound forms green, air-sensitive crystals, and like chromocene it has two unpaired electrons (Ht = 2.74 BM). This discovery was shortly followed by that of the curious complex, bis(pentadienyl)dinickel (12) which was prepared from NiCl2 and triethylaluminium in 1,4-pentadiene (122). The pentadienyl ligands in 12... 

Fig. 4. Relative orientations of pentadienyl ligands in bis(pentadienyl) metal complexes. 9 is the conformation angle.

In principle pentadienyls can bond to transition elements in at least three basic ways, tj3, and tjs (Fig. 1). These can be further subdivided when geometrical factors are considered. If r 5 coordination could be converted to rj3 orr/1, one or two coordination sites could become available at the metal center, and perhaps coordinate substrate molecules in catalytic processes. Little is known about the ability of pentadienyl complexes to act as catalysts. Bis(pentadienyl)iron derivatives apparently show naked iron activity in the oligomerization of olefins (144), resembling that exhibited by naked nickel (13). The pentadienyl groups are displaced from acyclic ferrocenes by PF3 to give Fe(PF3)5 in a way reminiscent of the formation of Ni(PF3)4 from bis(allyl)nickel (144). 

Bis(pentadienyl)metal Complexes with Additional Ligands... 

Recent advances in the techniques of photoelectron spectroscopy (7) are making it possible to observe ionization from incompletely filled shells of valence elctrons, such as the 3d shell in compounds of first-transition-series elements (2—4) and the 4/ shell in lanthanides (5, 6). It is certain that the study of such ionisations will give much information of interest to chemists. Unfortunately, however, the interpretation of spectra from open-shell molecules is more difficult than for closed-shell species, since, even in the simple one-electron approach to photoelectron spectra, each orbital shell may give rise to several states on ionisation (7). This phenomenon has been particularly studied in the ionisation of core electrons, where for example a molecule (or complex ion in the solid state) with initial spin Si can generate two distinct states, with spin S2=Si — or Si + on ionisation from a non-degenerate core level (8). The analogous effect in valence-shell ionisation was seen by Wertheim et al. in the 4/ band of lanthanide tri-fluorides, LnF3 (9). More recent spectra of lanthanide elements and compounds (6, 9), show a partial resolution of different orbital states, in addition to spin-multiplicity effects. Different orbital states have also been resolved in gas-phase photoelectron spectra of transition-metal sandwich compounds, such as bis-(rr-cyclo-pentadienyl) complexes (3, 4). 

Properties of bis(pentahaptocyclo-pentadienyl] complexes of the first-row transition metals... 

One example of a bonafide bis(alkyne) complex has recently been prepared. Reaction of the in situ generated olefin complex prepared by alkylation of ( -CsHs ZrC 50 with the diaryl alkyne in Equation (7) yields 253.130 In this structure, C-C coupling has not occurred, presumably a result of the steric strain associated with the zirconacyclo-pentadienyl fragment (Equation (7)). The solid-state structure further establishes the compound as a bis(alkyne) complex. Computational studies suggest that a Zr(iv) resonance structure is the most suitable representation of the compound. However, reaction of 253 with iodine in THF yields ( -CsHs Zrle 254 and the dialkyne starting material, suggesting that the zirconium center can act as a source of Zr(n) (Equation (8)). 

Other interesting uses of iron carbonyl complexes include the synthesis of N-substituted l//-l,2-diazepine (95) and the synthesis of cyclic hydrocarbons (96) via intramoleculai coupling of bis-pentadienyl iron tricarbonyl cations. 

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